DELTA PSC (Delta Piscium). The constellation Pisces, that of the celestial Fishes, is so large, dim,
and scattered that without the western "Circlet" it would be hard to recognize as any kind of
singular figure. Buried in the heart of the southern extension to
the southeast of the Great Square
of Pegasus is near-anonymous fourth
magnitude (but at 4.43 very close to fifth) Delta Psc. Obscure
though it may be it has its own stories to tell. Classed as a
cool, orange K (K5) giant, the
star is much like Aldebaran (Alpha
Tauri) though, at a distance of 311 light years (give or take 7),
almost five times farther away. From a variety of measures, the
temperature seems well-defined at 3975 Kelvin, which (to account
for a lot of infrared
radiation) with distance tells of a luminosity 380 times that
of the Sun. To produce that brightness at
that temperature requires a large size, calculated to be 42.1 times
that of the Sun, or roughly half as big as the orbit of Mercury.
Only two degrees north of the ecliptic, Delta Psc is regularly
occulted by the Moon. The time it takes for the star to be covered
by the easterly-moving lunar disk gives its angular size, which
with distance and a bit of trig gives the physical size. As is
often the case with large cool stars, the directly-determined
radius depends on color, or wavelength. In blue light, the radius
is 43.1 times solar, while infrared radiation gives 39.5 for an
average of 41.2, satisfyingly close to that found from temperature
and luminosity, showing that all the stellar parameters are in
order and pretty accurately known. A slow projected equatorial
rotation speed of 1.4 kilometers per second (similar to that of the
Sun) yields a rotation period that could be as long as 1500 days,
or just over four years. (Given that we have no idea about the
axial tilt, it could be a lot less.) Theory then leads to a mass
somewhere around 2-2.5 times that of the Sun (one source giving
1.5). Beginning life as a class A1 dwarf perhaps a billion or so
years ago, the star is most likely brightening as a giant for the
first time with a dead helium core. (Were it brightening for its
second "ascent" as a giant with a dead carbon/oxygen core, it would
probably be varying, at least slightly, and it isn't). Zipping
along relative to the Sun at 55 kilometers per second (4-5 times
normal), the metal content is consistently down by some 40 percent.
Nearly two minutes of arc away lies a possible companion, 13th magnitude Delta
Piscium B. If really bound to Delta A (which it may be as it's
held its position rather well for the past century), from its
brightness it is a K9 dwarf with a mass roughly half that of the
Sun. At a separation of at least 12,500 Astronomical Units it
would take at least 900,000 years to make a full orbit. More
likely, especially given the fragile nature of any such
gravitational bond, it is probably just another line-of-sight
coincidence.
Written by Jim Kaler 1/06/12. Return to STARS.